Question

How do skaters spin so fast? Try this: Sit on the rotating chair, and spin up to a good speed. Now, extend your arms and legs out, and pull them back in.

What do you observe as you do this? Explain what is happening in physics terms.

Where does the energy come from to make you spin faster? (Is that a good question to ask? Ask and answer a better one if you can!)

What else is changing as you spin faster or slower?

Please answer all parts, thank you!

Answer 1

Solution:

Obervation:

When the skater start spinning without stretching the hands and legs, he or she will spin with very high speed.

When the skater stretch hands and legs, he or she will spin with less speed.

Concept behind:

The physics concept behind the spinning of skaters is "Conservation of Angular momentum"

Angular momentum can be expressed as follows

where

When the skater either spinning with or without stretching the hands, his or her angular momentum will be conserved.

The only one parameter that will influence the speed of the skater is his or her "inertia"

As the "inertia" depends on the distribution of mass, the skater stretches his or her hands and legs to vary the distribution of his or her mass from the axis of rotation which inturn vary the rotation speed.

Therefore, when the skater bring back his or her hands and legs in, he gains energy due to the conservation of angular momentum.

As momentum is conserved the only parameter that will change when the skater spins faster or slower is the "inertia"